A classical line printer is a printer which prints one line (of usually 132 characters) at a time.

Usually fed with tractor feed printer paper, line printers were the first high-speed printers used in data processing. Nowadays, the classical, mechanical line printer has mostly been replaced by laser printers or line matrix/shuttle matrix printers.

Principle of operation:
Data is sent from the host to the device's buffer a line at a time. A print chain (imagine a cross between a bicycle chain and the types of a typewriter) moves in front of a hammer bank with one hammer per print position. Whenever the correct letter on the chain is in front of one of the hammers, the hammer hits the character on the chain, which in turn will hit the ribbon, creating an imprint on the paper. As soon as the line is completed, paper is advanced by one line.

The whole process is rather noisy, print quality is limited and except for bolding, and underscoring there is no real variation in the (fixed-width) fonts.

Speed is measured in LPM (lines per minute)
lpt, lpd, lpr and similar terms all have their origin in line printer.

Form control and control characters are not incuded in this wu - yet...

Line printers were sort of half way between the daisy wheel printer and the dot matrix printer in that they had the speed of the dot matrix (faster actually) and nearly the quality of the daisy wheel.

Actually, there were really several different types of line printers, with different characteristics. The description by McSnarf fits the old IBM chain/train line printers which were available as early as 1958 and its descendents which were available well into the 1970's. Early versions sounded like a machine gun, but later versions had enough sound insulation to just sound like lots of rapid tapping.

One of the most famous line printers is the Printronix printer, which has been in the background of many movies, including Sneakers. Unlike the IBM line printers, the Printronix line printer was much closer to a dot matrix printer in quality and mechanism. It had a print head consisting of 132 pins in a horizontal line. Pins needed would fire simultaneously and then the whole print head would shift over one pixel and repeat (7 times) and then advance the paper one pixel to print the next line. The printronix had a character set in rom, but could also print raster graphics. (I wrote a ghostscript driver, but never published it.) The technical manual had big warnings about not exceeding the duty cycle of the print head drivers or it would overheat. This whole process made a nice satisfying chung chung noise and spit out your print job out faster than you could cancel it.

Both IBM and Printronix still make and sell line printers, but they are probably based on a totally different technology. (I have not seen a modern one.) The common characteristic would be high speed and measuring that speed in lines per minute.

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